Spin flip excitations in bilayer quantum Hall ferromagnets

Bilayer quantum Hall systems at ?=1 are believed to have a ground state with spontaneous spin-polarization and spontaneous interlayer phase coherence. In this paper, we address the spectrum of spin-flip collective excitations using a generalized lattice spin model that appeals to the quantum Hall gap to justify neglect of charge fluctuations. We find that the long wavelength spin-flip excitation spectrum consists of the global Larmor spin-wave mode and a higher energy mode in which spin-waves in the two-layers are out of phase. We address the dependence of the energy splitting of the two modes on quantum fluctuations in the bilayer ground state.